An electrophotographic image forming apparatus forms an image using an electrostatic image developing toner (hereafter, it may be simply called as a toner) on a transfer medium such as paper, then, the formed toner image is fixed.
As a fixing method, a heat roller fixing method is widely used. In this method, the transfer medium after formed an image thereon is passed between a heat roller and a pressure roller. The heat roller is required to have a high heat capacity in order to obtain an excellent fixing property, namely, to obtain a high toner adhesion property on the transfer medium.
From the viewpoint of preventing warming of a global environment in recent years, there is an increasing demand for energy saving with respect to an electrophotographic image forming apparatus. Therefore, in an image forming apparatus employing a heat roller fixing system, many investigations have been made for achieving a low-temperature fixing toner in order to reduce the amount of heat required for fixing.
A most representative toner aiming at the low-temperature fixing is a toner using a crystalline resin.
For example, it was proposed a toner containing a crystalline polyester resin as a threadlike crystalline structure. This toner enabled to achieve a sharp-melting property of the crystalline polyester resin and improved low-temperature fixing (Patent document 1: JP-A No. 2013-257415).
Further, it was proposed a toner having a crystal line polyester resin finely dispersed as a domain phase of an average size of 300 nm or less in an amorphous resin. This structure was achieved by adjusting a content of a carboxy group in the amorphous resin and a content of an ester group in the crystalline polyester resin (Patent document 2: JP-A. No. 2014-185194).
The crystalline polyester resin in this toner will promote compatibility during the heat fixing. In addition, it has a small degree of crystallization and a small amount of dispersion in the degree of crystallization. As a result, the image formed with this toner will have an improved uniform glossiness.
On the other hand, as coloring agents in yellow toners, there are used pigments such as: C. I. Pigment Yellow 93, C. I. Pigment Yellow 155, C. I. Pigment Yellow 180, C. I. Solvent Yellow 93, and C. I. Solvent Yellow 163 (Patent documents 3 and 4: JP-A Nos. 2000-75552 and 2006-313302).
However, these coloring agents have insufficient coloring power when used alone. In order to obtain a required coloring power, an added amount of the coloring agent has to be increased.
However, when the added amount of the coloring agent is increased, the incorporating capacity of the coloring agent in the toner will be lowered, and there are produced problems that the charging property is decreased and the toner is easily scattered. Some pigments contain an aromatic amine, and they will emit odor of an aromatic amine evaporated during thermal fixing when they are contained with an increased amount (for example, refer to Patent document 5: JP-A No. 2004-109310). In addition, when a toner contains too much amount of pigment, the toner will hardly decrease elasticity by the filler effect. This will deteriorate low-temperature fixability.
When an added amount of a toner to a paper is increased, it may be obtained a required coloring power without increasing an amount of an added pigment. However, this will cause increased cost and easy scattering by an increased added amount toner.